Angiography is a common method used to represent blood vessels based on diagnostic imaging methods, such as X-ray or Magnetic Resonance Tomography (MRT). For an improved representation of the vessels under examination, Digital Subtraction Angiography (DSA) has been developed. DSA is a fluoroscopy technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment. Images are produced by subtracting a ‘pre-contrast image’ or the mask from subsequent images acquired after the contrast medium has been introduced into a structure or tissue of interest. These images can be used to provide time-resolved or time-varying information that shows the development of the structure or tissue of interest over time.
In current clinical practice, time-resolved information is generally only available in two dimensions. Typically, the surgeon has to perform a two-dimensional (2D) to three-dimensional (3D) mental conversion from the 2D projection images to 3D anatomy in order to assess and diagnose vascular pathologies and blood flow abnormalities. The filling of the vasculature changes from frame to frame, leaving the surgeon with the difficult task of interpreting 3D filling from varying 2D snapshots. Regardless of acquisition/viewing angle, vessel segments that are overlapping and/or obscured may therefore be compromised, leading to potentially missing image information or incorrect diagnosis. Problems include, for example, vessel overlap or vessels running orthogonal to the detector plane.
Vascular filling may be visualized using single-plane or bi-plane 2D DSA image(s). Existing methods may provide acceptable results, but struggle with complex vasculature and occluded vessels, introduction of premature vessel filling, and fluctuations in vessel filling. Some traditional techniques use data from static angles instead of the acquisition sequences themselves, which can lead to additional radiation exposure for the patient as well as inaccuracies arising from a corresponding need for highly accurate image registration steps. Other methods are based on simplifying assumptions (i.e., simplified models) concerning the patient's physiology (e.g., periodic cardiac activity) as well as the transport of blood, and mixture of blood and contrast agent, through the patient's vasculature. These may lead to reconstructed flow results that deviate from real flow patterns.
Recent years have seen the introduction of methodologies for non-time-resolved 3D-DSA. In one method, a mask projection image sequence is first acquired during a rotational scan of the angiographic device, followed by a sequence of rotational fill projection images acquired after the introduction of contrast agent. The mask projection images are subtracted from the fill projection images to generate projection image data that displays a subject's vascular anatomy acquired at different viewing angles. Using 3D reconstruction techniques, a static volumetric dataset of a subject's vasculature can be created. This static reconstruction does not, however, include the change recorded in the acquisition temporal sequence.